1. Field of the Invention
The present invention relates to the art of electrophotographic recording system and more particularly to an apparatus forming electrostatic latent images on a recording medium according to information received in an electronic signal and developing the images one by one with a plurality of color developing devices so that the images are superimposed one upon another and a composite color image is produced on the recording medium.
2. Description of the Related Art
In recent years, with the development of color plotter systems and printers, there have been increasing demands for providing picture images in color printing being as fine as those in color photography.
There have been known monocolor electrophotographic recording and multicolor electrophotographic recording as the recording methods of recording color images. In the monocolor recording, development is made immediately after a latent image is formed. In the multicolor recording, however, two or more latent images must be superposed independently. Therefore, in order for the different color component images to be correctly superposed so as not to produce color fringes and color registration errors and thereby to assure the production of a high resolution color print, there must be provided some means for registration.
The problem of registration of color images and means for solving the problem are stated and proposed in U.S. Pat. No. 4,007,489. In this patent, a series of recording/developing stations are provided, each station having one recording head and one developing device for each color. The registration of the component images is achieved by means of a cross mark for printing placed on the surface of the recording medium, which is adapted to be sensed by a series of sensors before each recording/developing station. However, the use of a single registration mark and multiple recording stations has several disadvantages. That is, due to differences of time setting for the latent images formed and of physical parameters of the different recording head structure, displacement of the recording medium is easily produced in both longitudinal and lateral directions of the recording medium. And, as the recording medium moves within the system from station to station, position of the recording medium readily changes. Furthermore, the recording medium changes in size, that is, produces expansion or shrinkage in both lateral and longitudinal directions with reference to the length. Such expansion or shrinkage is sufficient to result in lower resolution of the composite image due to color fringes and color registration errors and to result in misalignment of the component images.
As a solution to the above mentioned problems, there is known an art as described in U.S. Pat. No. 4,569,584. The apparatus of this patent is provided with a recording station including recording electrode means for forming latent images on a recording medium and a plurality of developing means disposed adjoining either one side or both sides of the recording station, each developing means including a color developing device for forming a color component image to constitute a composite image.
According to this patent, the recording medium is transported within the apparatus in opposite directions. The direction reversal of the recording medium is made at least once after a first component latent image has been formed by the recording head and its color component image has been developed, so that a second component latent image may be formed superimposed on the thus developed first component image, and this color component image is then developed. The process may be repeated the same number of times as the number of desired color component images.
Furthermore, according to the description in this U.S. Pat. No. 4,569,584, registration means associated with transport of the recording medium and formation of each of the component images is provided therein so that the component images of various colors are superimposed on one another with sufficient accuracy.
The registration means includes recording means within the recording station to form aligned latent tracking indexes, which are subsequently developed to be visualized, on the recording medium outside the field of the composite color image. The tracking indexes may be printed on the recording medium when it is manufactured, but this considerably increases the cost of the recording media as consumables and is not preferable from the viewpoint of cost effectiveness.
As means associated with the transport of the recording medium, two kinds of photoelectric sensors are used to sense the aligned tracking indexes to provide electrical signals representative of information about dimensions of the recording medium in both the longitudinal and lateral directions, and thereby, adjustment of the component latent image in both the longitudinal and lateral directions is made achievable.
While the component latent image is being formed, relative translation between the recording medium and the recording head is provided. This is achievable in several ways. First, the supply roll from which the recording medium is let out into the electrophotographic apparatus etc. is laterally translated with reference to the medium path within the apparatus. Second, the recording head including the electrode means is laterally translated relative to the recording medium. Third, the recording head is rotated with reference to the path of the recording medium within the apparatus. Further, the traveling speed of the recording medium along its path is controlled, or the frequency for energizing the electrode means is controlled, based upon the sensed information, so that more data lines or less data lines may be included in the component latent image formed.
The tracking indexes comprise a series of aligned registration marks having the same spacing and width and disposed adjoining one edge or both edges of the recording medium. The registration marks are preceded by a plurality of aligned initializing marks in a particular recording medium portion, which have a different geometric shape from the registration marks, for example, a different mark width. The point of change from the last narrower initializing mark to the first wider registration mark is indicative of the starting location, termed START PLOT, for each component latent image.
Lateral and longitudinal demensional changes in the medium derived from observation of an aligned row of registration marks is indicative of changes in length, either expansion or shrinkage, of the recording medium section under observation. In this regard, coarse correction for lateral alignment of the recording medium relative to the recording head due to medium shifting in the medium path can be accomplished by the lateral translation of the medium supply roll, while fine correction for lateral latent image alignment due to medium expansion or shrinkage can be accomplished by the lateral translation of the recording head to recenter the head relative to the medium, or by the lateral shifting of the energization of the electrode means, i.e., the lateral start point of the latent image formation, as described in U.S. Pat. No. 4,007,489.
Alternatively, a tracking line adjacent to and parallel with the aligned row of registration marks at both edges of the recording medium may be employed for lateral head corrective translation.
However, the color image recording apparatus structured as above has disadvantages. Since there are provided a series of registration marks and tracking lines along both edges of the recording medium, the portion where an image is practically unable to be recorded becomes larger. Furthermore, since the registration in the direction of transport of the recording medium is controlled by the recording timing provided based on measurement of the pitch of the registration marks, such errors that are produced while the recording medium is transport, e.g., slippage of the transport roller, will produce a displacement that remains in the recorded image throughout the subsequent recording process and as such will cause color registration errors when a plurality of colors are superimposed. Still furthermore, since the alignment in the direction perpendicular to the direction of transport of the recording medium is accomplished such that the tracking lines on the left and right are respectively watched by two optical sensors and the control is made so that the outputs of both of the optical sensors may be balanced, if there is a broken portion or uneven density in the tracking line, the position of the image will be shifted in spite of there being no meandering movement or expansion or shrinkage of the recording medium, and thereby, color registration errors will be produced. There were such problems in the prior art.